U.S. patent number 5,213,617 [Application Number 07/714,811] was granted by the patent office on 1993-05-25 for primer for silicone substrates.
This patent grant is currently assigned to Dow Corning Corporation. Invention is credited to John D. Blizzard.
United States Patent |
5,213,617 |
Blizzard |
May 25, 1993 |
Primer for silicone substrates
Abstract
A primer composition, and a method for its use to improve
adhesion between a silicone substrate and a subsequently applied
paint film, is disclosed. The primer composition consists
essentially of a mixture of an alkoxysilane, an organotitanate and
a tin salt of a carboxylic acid, each component being present in a
specific critical molar proportion.
Inventors: |
Blizzard; John D. (Bay County,
MI) |
Assignee: |
Dow Corning Corporation
(Midland, MI)
|
Family
ID: |
24871559 |
Appl.
No.: |
07/714,811 |
Filed: |
June 13, 1991 |
Current U.S.
Class: |
106/287.13;
106/287.14; 106/243; 106/287.15 |
Current CPC
Class: |
C08G
77/58 (20130101); C08G 79/14 (20130101); C09D
183/14 (20130101); C08J 7/043 (20200101); C08J
7/0427 (20200101); C08J 2383/04 (20130101) |
Current International
Class: |
C08G
79/14 (20060101); C08G 77/00 (20060101); C08G
79/00 (20060101); C08G 77/58 (20060101); C09D
183/14 (20060101); C08J 7/04 (20060101); C08J
7/00 (20060101); C08K 005/54 () |
Field of
Search: |
;106/287.13,287.14,287.15,287.16,287.19,243 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Brunsman; David
Attorney, Agent or Firm: Weitz; Alexander
Claims
I claim:
1. A primer composition consisting essentially of a mixture of
(I) from 25 to 75 mole percent of at least one alkoxysilane having
the general formula
wherein R is a monovalent organic moiety selected from the group
consisting of an alkyl group having 1 to 6 carbon atoms, an allyl
radical, a vinyl radical, an epoxy functional organic group and an
acryl-functional organic group, R' is an alkyl radical having 1 to
3 carbon atoms and x is an integer having a value of 1 to 3
inclusive;
(II) from 13 to 60 mole percent of an organotitanate having the
formula
wherein R'' is an alkyl radical having 1 to 8 carbon atoms; and
(III) from 12 to 60 mole percent of a tin salt of a carboxylic acid
having 2 to 18 carbon atoms.
2. The composition according to claim 1, wherein x of said
alkoxysilane (I) is 1.
3. The composition according to claim 2, wherein R' of said
alkoxysilane (I) is a methyl radical.
4. The composition according to claim 3, wherein said alkoxysilane
(I) is selected from the group consisting of allyltrimethoxysilane,
vinyltrimethoxysilane, methyl trimethoxysilane,
gamma-glycidoxypropyltrimethoxysilane,
acryloxypropyltrimethoxysilane and
methacryloxypropyltrimethoxysilane.
5. The composition according to claim 4, wherein said
organotitanate (II) is selected from the group consisting of
tetrabutyl titanate and tetraisopropyl titanate.
6. The composition according to claim 5, wherein said tin salt
(III) is selected from the group consisting of dibutyltin diacetate
and dibutyltin dilaurate.
7. The composition according to claim 6, wherein said alkoxysilane
(I) is allyltrimethoxysilane and the molar percentages of said
alkoxysilane (I), said organotitanate (II) and said tin salt (III)
are 48 to 65, 20 to 36 and 15 to 31, respectively.
8. A primer composition consisting essentially of:
(i) a mixture of
(I) from 25 to 75 mole percent of at least one alkoxysilane having
the general formula
wherein R is a monovalent organic moiety selected from the group
consisting of an alkyl group having 1 to 6 carbon atoms, an allyl
radical, a vinyl radical, an epoxy-functional organic group and an
acryl-functional organic group, R' is an alkyl radical having 1 to
3 carbon atoms and x is an integer having a value of 1 to 3
inclusive;
(II) from 13 to 60 mole percent of an organotitanate having the
formula
wherein R'' is an alkyl radical having 1 to 8 carbon atoms; and
(III) from 12 to 60 mole percent of a tin salt of a carboxylic acid
having 2 to 18 carbon atoms, and
(ii) from 90 to 99.5 weight percent of an organic solvent based on
the weight of said mixture (i).
9. The composition according to claim 8, wherein x of said
alkoxysilane (I) is 1.
10. The composition according to claim 9, wherein R' of said
alkoxysilane (I) is a methyl radical.
11. The composition according to claim 10, wherein said
alkoxysilane (I) is selected from the group consisting of
allyltrimethoxysilane, vinyltrimethoxysilane, methyl
trimethoxysilane, gamma-glycidoxypropyltrimethoxysilane,
acryloxypropyltrimethoxysilane and
methacryloxypropyltrimethoxysilane.
12. The composition according to claim 11, wherein said
organotitanate (II) is selected from the group consisting of
tetrabutyl titanate and tetraisopropyl titanate.
13. The composition according to claim 12, wherein said tin salt
(III) is selected from the group consisting of dibutyltin diacetate
and dibutyltin dilaurate.
14. The composition according to claim 13, wherein said
alkoxysilane (I) is allyltrimethoxysilane and the molar percentages
of said alkoxysilane (I), said organotitanate (II) and said tin
salt (III) in said mixture (i) are 48 to 65, 20 to 36 and to 31,
respectively.
Description
FIELD OF THE INVENTION
The present invention relates to a primer composition for silicone
substrates and a method for its use to improve adhesion between the
primed substrate and a subsequently applied overcoat or paint film.
More particularly, the invention relates to a primer composition
consisting essentially of a mixture of an alkoxysilane, an
organotitanate and a tin salt of a carboxylic acid, each component
being present in a specific critical proportion.
BACKGROUND OF THE INVENTION
Various substrates, such as metals, glass and plastics, require the
application of a primer before they can be coated with a
conventional paint or other coating formulation. Such primer
compositions have often relied on one or more organosilane
compounds to promote the adhesion between substrate and coating.
Thus, for example, Finzel, in U.S. Pat. No. 4,654,236, teaches a
method for priming coil coated substrates with a solution of a
tetraorganosilane and a tetraalkyltitanate. The primed surfaces can
be subsequently painted to provide surfaces which are as durable as
the original coil coatings. Similar organosilanes can also be
employed in various compositions for purposes other than priming
substrates. For example, U.S. Pat. No. 4,846,886 to Fey et al.
discloses compositions wherein an improved water repellent
composition for porous substrates is formed from an
alkylalkoxysilane, a carrier, a water beading agent and a metal
salt catalyst.
Of concern herein is the particular difficulty of painting or
overcoating a silicone substrate. This operation is particularly
troublesome since silicone and silicone polymer substrates
characteristically exhibit very low surface energies. They are thus
better known in the art for their abhesive nature and typically
tend to release most coating compositions applied thereto. Since it
is often desirable to paint or otherwise overcoat a silicone
substrate (e.g., for decorative purposes. abrasion/erosion
resistance, printing, marking) many attempts have been made over
the years to develop satisfactory primers for this application. The
following disclosures represent the relevant art of this type known
to applicant.
Young, in U.S. Pat. No. 3,671,483, discovered that a solution
containing a tetraalkyltitanate and a particular silane which has
at least one group of the formula --CH.sub.2 CH.sub.2
--Si(OOCCH.sub.3).sub.3 in its molecule is useful as a primer in
adhering a silicone rubber to epoxy and polyurethane.
In U.S. Pat. No. 3,677,998, Young also teaches that a solution of
an organotitanate, tetra(methoxyethoxy)silane, a methylsiloxane
resin and either acetic acid or an organotriacetoxysilane is useful
as a primer in adhering a silicone rubber to a metal surface.
Hirai et al., in U.S. Pat. No. 4,598,134, teach a primer
composition for heat curable silicone rubber consisting essentially
of at least one epoxy-functional organosilicon compound, a
trialkoxysilane having either two silicon-bonded hydrogens or two
unsaturated groups or a partial hydrolyzate of said
trialkoxysilane, and an organotitanate ester.
Saito et al., in U.S. Pat. No. 4,681,636 disclose a primer
composition particularly suitable for bonding fluorosilicone rubber
to a substrate. This primer comprises a solution of a t-butyl
peroxy-functional silane, a reactive organosilicon compound having
at least two alkoxy groups and an organotitanate ester. In another
disclosure to Saito et al., U.S. Pat. No. 4,749,741, primers
suitable for promoting adhesion between silicone rubber and
plastics or metals are taught. These primers are similar to their
previous systems and comprise a solution of an unsaturated
organotitanate ester and either an organosilicon compound having at
least two alkoxy groups or an organohydrogensiloxane (or
silane).
In U.S. Pat. No. 4,704,419, Fukayama et al. teach primer
compositions which improve adhesion of paints to cured silicone
rubber and comprise a solution of an amine-functional
dialkoxysilane and a liquid hydroxyl terminated polyorganosiloxane,
optionally further comprising a catalyst.
Meddaugh, in U.S. Pat. No. 4,923,946, discloses a primer for
moisture-cured silicone sealants which imparts paintable surfaces
thereto. The primers disclosed contain a solution of a hydrolyzate
of an aminoalkyl-substituted trialkoxysilane and a carboxylic acid
salt of tin.
A method for bonding to a silicone rubber is disclosed in Japanese
Kokai Patent No. 59[1984]-48768 to Toray Industries, Inc. Here, a
waterless lithographic plate having a photosensitive layer, an
adhesive layer and a silicone rubber layer is described wherein the
adhesive layer contains an organic titanate. Printing plates of
this invention are stated have a strong adhesion between the
photosensitive layer and the silicone rubber layer.
In addition to the above described art, the following United States
patents teach various primer compositions which promote adhesion to
silicone surfaces: 4,431,472 to Hohl et al., 4,122,127 to Mikami et
al., 4,534,815 to Hamada et al., 4,546,018 to Ryuzo et al.,
4,133,938 to Bingham, 4,147,685 to Smith, 4,332,844 to Hamada et
al., 3,619,256 to Pepe et al., 4,808,483 to Nakasuji et al.,
4,436,787 to Mikami et al. and 4,401,500 to Hamada et al.
SUMMARY OF THE INVENTION
It has now been found that certain blends of an alkoxysilane, an
organotitanate and a tin salt of a carboxylic acid can act as a
superior primer composition for certain silicone substrates which
are subsequently to be overcoated or painted with a silicone or
organic composition, thereby providing greatly improved adhesion
between the silicone substrate and the overcoat film. Surprisingly,
it has been observed that this marked improvement in adhesion is
obtained only when the above named components are present in the
critical molar ratios described infra. Thus, for example, a
preferred water repellant composition disclosed by Fey et al.,
cited supra, consists of 85 weight percent
isobutyltrimethoxysilane, 10 weight % tetraisopropyltitanate and 5
weight % dibutyltindilaurate. This composition, although similar to
the primers of the instant invention, is not within the critical
range discovered by applicant and has not been found to be an
effective primer for silicone substrates. More to the point, Fey et
al. do not suggest the use of their water repellant compositions as
primers for silicone substrates. It is thus the discovery of the
use of such narrowly defined compositions to prime certain silicone
substrates, and the discovery of the apparently synergistic range
of the necessary ingredients, which is at the heart of the present
invention.
The instant invention therefore relates to a primer composition
consisting essentially of a mixture of (I) from 25 to 75 mole
percent of at least one alkoxysilane having the general formula
wherein R is a monovalent organic moiety selected from the group
consisting of an alkyl group having 1 to 6 carbon atoms, an allyl
radical, a vinyl radical, an epoxy-functional organic group and an
acryl-functional organic group. R' is an alkyl radical having 1 to
3 carbon atoms and x is an integer having a value of 1 to 3
inclusive; (II) from 13 to 60 mole percent of an organotitanate
having the formula
wherein R'' is an alkyl radical having 1 to 8 carbon atoms; and
(III) from 12 to 60 mole percent of a tin salt of a carboxylic acid
having 2 to 18 carbon atoms.
The present invention further relates to a method for overcoating a
silicone substrate with a paint comprising first coating said
silicone substrate with the above described primer, thereby forming
a primed silicone substrate, and then overcoating said primed
silicone substrate with said paint.
DETAILED DESCRIPTION OF THE INVENTION
Component (I) of the primer compositions of the present invention
is an alkoxysilane, or a mixture of alkoxysilanes, each
alkoxysilane having the general formula
wherein R is a monovalent organic moiety independently selected
from the group consisting of an alkyl group having 1 to 6 carbon
atoms, an allyl radical, a vinyl radical, an epoxy-functional
organic group and an acryl-functional organic group, R' is an alkyl
radical having 1 to 3 carbon atoms and x is an integer having a
value of 1 to 3 inclusive. It is preferred that component (I) is a
trialkoxysilane (i.e., x=1), most preferably a
trimethoxysilane.
For the purposes of the present invention, when R is the above
mentioned epoxy-functional organic group, it is attached to the
silicon atom by a silicon-carbon bond, the organic portion acting
only as a connecting group between the epoxy functionality and the
silicon atom of the silane. The nature of the connecting group is
thus not critical as long as it contains no more than about 6
carbon atoms and it contains no other functionality which is
reactive with components (II) and (III) of the invention, described
infra. As will be apparent to the skilled artisan, this connecting
group must also not adversely affect the method of the present
invention wherein a paint or other coating is applied to a silicone
substrate primed with the compositions of the invention. Examples
of suitable connecting groups include alkylene groups, such as
trimethylene, tetramethylene and hexamethylene, and oxygen-linked
alkylene groups, such as oxypropylene and oxybutylene. Specific
epoxy-functional alkoxysilanes include
glycidoxypropyldimethylmethoxysilane and
glycidoxypropylmethyl-di-isopropoxysilane. A particularly preferred
epoxy-functional alkoxysilane of the invention is
gamma-glycidoxypropyltrimethoxysilane.
Similarly, when R is the above mentioned acryl-functional organic
group, it is attached to the silicon atom through a silicon-carbon
bond wherein the nature of the organic connecting group is again
not critical as long as it contains no more than about 6 carbon
atoms and does not contain any functionality which can react
adversely with the other components of the compositions and method
of the instant invention. Thus, the connecting groups described
above for the epoxy-functional organic group are also suitable in
the acryl-functional silanes. As used herein, the term "acryl"
refers to a moiety having the structure
in which Q is selected from the group consisting of hydrogen and an
alkyl radical having 1 to 6 carbon atoms. Preferred alkoxysilanes
having such acryl functionality include
acryloxypropyltrimethoxysilane and
methacryloxypropyltrimethoxysilane.
Specific examples of other alkoxysilanes which are suitable for use
as component (I) include methyltrimethoxysilane,
vinyltrimethoxysilane, allyltrimethoxysilane,
butyltrimethoxysilane, hexyltrimethoxysilane, vinyltriethoxysilane,
allyltriethoxysilane, methyltriethoxysilane,
dimethyldimethoxysilane, methylvinyldimethoxysilane and
methylallyldimethoxysilane.
Component (II) of the present invention is an organotitanate having
the formula
wherein R'' is an independently selected alkyl radical having 1 to
carbon atoms. Specific examples of suitable organotitanates include
tetrabutyl titanate, tetraisopropyl titanate, tetramethyl titanate,
ethylmethyldibutyl titanate, tetraoctyl titanate and
tetraethylhexyl titanate. In preparing the primer compositions of
the present invention, it is preferred that component (II) is
either tetra(n-)butyl titanate or tetraisopropyl titanate.
Surprisingly, it has been found that chelated organotitanates, such
as titanium isopropoxybis(acetylacetonate), result in the formation
of undesirable precipitates when mixed with components (II) and
(III), described infra. Therefore, even though such titanium
compounds have been successfully employed in various primer
compositions of the prior art, they are not within the scope of the
present invention.
Component (III) of the present invention is a tin (II) or tin (IV)
salt of at least one carboxylic acid having 2 to 18 carbon atoms,
the remaining valences of the tin being satisfied by a moiety
selected from the group consisting of alkyl radicals having 1 to 4
carbon atoms and a phenyl radical. Examples of suitable tin (IV)
compounds include dibutyltin diacetate, dibutyltin dilaurate and
tin tripropyl acetate. Examples of tin (II) compounds include
stannous octoate, stannous oxalate, stannous caprylate, stannous
2-ethylhexoate, stannous naphthanate, stannous oleate, stannous
palmitate and stannous stearate.
The above described components (I), (II) and (III) of the present
invention are well known in the art, many of them being
commercially available, and no further description thereof is
considered necessary to enable one of ordinary skill in the art to
practice the invention.
In order to prepare the primer compositions of the present
invention, from about 25 to about 75 mole percent of component (1),
from about 13 to about 60 mole percent of component (II) and from
about 12 to about 60 mole percent of component (III) are thoroughly
blended to form a homogeneous dispersion or solution. It has been
found that primer compositions outside these critical ranges have a
significantly reduced ability to promote adhesion between a
silicone substrate coated with the primer composition and a
subsequently applied paint or overcoat layer. In highly preferred
embodiments, the relative molar percentages of components (I), (II)
and (III) are 48 to 65, 20 to 36 and 15 to 31, respectively.
Although the above described primer composition can be applied
(i.e., coated) directly to a silicone substrate at 100% solids, it
is preferred to dilute the composition with one or more organic
solvents in order to reduce application viscosity and limit the
thickness of the primer coat. Examples of suitable solvents for
this purpose are hexane, heptane, toluene, xylene, mineral spirits
and cyclic siloxanes, heptane being preferred. When a solvent or
solvent mixture is employed, it is preferred that from about 0.5 to
about 10 weight percent of the above described primer composition
is thoroughly dispersed therein.
In addition to the solvent, it is contemplated that the primer
compositions of the present invention can further comprise minor
portions (e.g., up to about 5 parts by weight per 100 parts by
weight of said component I, II and III) of dyes, pigments, flow
additive, and the like.
In order to use the above described primer composition according to
the method of the present invention, the primer composition may
first be applied to a silicone substrate by the usual techniques
practiced in the art, such as dip coating, spray painting, knife
coating, brush coating, inter alia. As used herein, the term
"silicone substrate" is intended to encompass both silicone
homopolymer compositions and organic-silicone copolymer
compositions and includes such substrates as silicone rubbers,
silicone sealants, silicone resins, silicone-alkyd copolymer
compositions, silicone-epoxy copolymer compositions,
silicone-urethane copolymer compositions, and the like. However, it
has been observed that not all types of silicone substrates can be
effectively primed with the compositions of the invention. A
preferred silicone substrate which may be primed with the instant
compositions to illustrate the unexpectedly superior performance
thereof is a cured product based on silicone systems taught in U.S.
Pat. No. 4,537,829 to Blizzard et al., which patent is assigned to
the assignee of the present invention and is hereby incorporated by
reference. Briefly stated, these curable compositions comprise a
blend of (A) from 1 to 200 parts by weight of a liquid
organosilicon resin, (B) 100 parts by weight of a vinyl or
hydroxyl-functional organosilicon polymer, (C) from 1 to 50 parts
by weight of a vinylated organosilicon resin and (D) from 0.00003
to 0.035 parts by weight of a hydrosilylation catalyst. The above
named component (A) is a reaction product of (i) an organosilicon
resin consisting essentially of triorganosiloxy units and SiO.sub.2
units in a molar ratio of 0.6 to 0.9 and (ii) a
polyorganohydrogensiloxane. Component (B) is an organosilicon
polymer having the formula
wherein each R''' is selected from the group consisting of
monovalent hydrocarbon radicals, each R'' is selected from the
group consisting of OH radicals and R''' radicals, y has a value of
0 to 5000, there being on average at least two radicals of the
organosilicon polymer selected from the group consisting of vinyl
radicals and hydroxyl radicals. Component (C) in the above
composition is an organosilicon resin consisting essentially of
Me.sub.2 (CH.sub.2 .dbd.CH)SiO1/2, Me.sub.3 SiO.sub.1/2 units and
SiO.sub.4/2 units in a molar ratio of 0.1:0.6:1 to 0.25:0.9:1.
respectively, wherein Me hereinafter denotes a methyl radical.
Another preferred silicone substrate is a cured elastomer based on
a high molecular weight vinyl-functional silicone gum (e.g., one
having a viscosity of at least about 10.sup.4 Poise at 25.degree.
C.) which contains from about 10 to about 40 parts by weight of a
silica filler having a surface area between about 50 and 300
m.sup.2 /g. These materials are well known in the art, are
available commercially and are generally cured with the aid of a
peroxide catalyst.
After the primer has dried on the silicone substrate for about 15
minutes to about 24 hours and essentially all of the dilution
solvent has evaporated therefrom, the primed silicone substrate is
overcoated or painted with a conventional coating or paint
composition. The latter material may be selected from organic or
silicone systems or compositions based on copolymers thereof. Thus,
for example, the overcoat may comprise a formulated silicone resin
paint, an alkyd paint, a polyurethane paint or a silicone-alkyd
paint, among others. After the paint or coating has been applied
and dried and/or cured onto the primed silicone substrate, an
improved degree of adhesion between the paint or coating and the
silicone substrate is obtained. A paint which derives a particular
advantage in this manner is one based on a silicone-alkyd copolymer
and has been effectively employed as a maintenance coating for
naval vessels, wherein durability and good adhesion to the
overcoated substrate is critical. Other paints or coatings can be
based on, e.g., polyimide or epoxy polymers.
EXAMPLES
The following examples are presented to further illustrate the
primer compositions and method of the invention, but are not to be
construed as limiting the invention, which is delineated in the
appended claims. All parts and percentages in the examples are on a
weight basis and all measurements were obtained at 25.degree. C.,
unless indicated to the contrary.
The following materials, listed alphabetically for ease of
reference, were employed in the examples:
ATMS=Allyltrimethoxysilane.
DBTDA=dibutyltin diacetate.
DBTDL=dibutyltin dilaurate.
IBTMS=isobutyltrimethoxysilane.
MTMS=methyltrimethoxysilane.
TBT=tetra(n-)butyl titanate.
TPT=tetraisopropyl titanate.
VTMS=vinyltrimethoxysilane.
Z-6020=An alkoxysilane consisting essentially of
N-(beta-aminoethyl)-gamma-aminopropyltrimethoxysilane; marketed by
the Dow Corning Corp., Midland, Mich.
Z-6030=An alkoxysilane consisting essentially of
gamma-methacryloxypropyltrimethoxysilane; marketed by the Dow
Corning Corp.
Z-6040=An alkoxysilane consisting essentially of
gamma-glycidoxypropyltrimethoxysilane; marketed by the Dow Corning
Corp.
EXAMPLES 1-25
Primer compositions of the invention and comparative compositions
were prepared by thoroughly mixing the components (I), (II) and
(III) shown in Table 1 in the molar ratios indicated. Each primer
composition was diluted with heptane to provide an approximately 10
weight percent dispersion therein, the following being exceptions:
the compositions of Examples 9-11 and 15 as well as (Comparative)
Examples 22-24 were diluted to form 4% dispersions in heptane and
Examples 13-14 were diluted to form 12% dispersions in heptane.
A curable silicone rubber coating formulation was prepared by
thoroughly mixing equal weights of the following two components
according to the teachings of above cited U.S. Pat. No. 4,537,829
to Blizzard et al. The first component consisted of a mixture of 82
weight percent of a dimethylvinylsiloxy endblocked
polydimethylsiloxane having a viscosity of about 3 Pa's and 18
weight percent of a benzene soluble resin copolymer of
triorganosiloxy units and SiO.sub.2 units in the molar ratio of
about 0.7 mole of triorganosiloxy units per mole of SiO.sub.2 units
where the triorganosiloxy units were trimethylsiloxy units and
dimethylvinylsiloxy units and the resin copolymer contained about 2
weight percent silicon-bonded vinyl radicals. The second component
of the silicone coating composition was a blend of 91.3 weight
percent of a liquid organosilicon resin as shown in Example 1 of
U.S. Pat. No. 4,537,829 (at column 8, lines 23-44), 0.5 weight
percent of 3.5 dimethyl-1-hexyn-3-ol and 8.2 weight percent of a
mixture of methylvinylcyclosiloxanes.
The above curable silicone rubber mixture was catalyzed with a
platinum complex hydrosilation catalyst (53 ppm platinum based on
the weight of the total coating mixture). The catalyzed system was
coated onto steel panels and cured thereon at room temperature for
at least 24 hours.
Heptane dispersions of the primer composition described in Table 1
were wiped onto the panels coated with the cured silicone rubber
using a cheesecloth applicator and the resulting film was allowed
to air dry for about 30 minutes. At this point, the primed panels
were overcoated with a flat grey silicone-alkyd paint obtained from
Seagrave Coatings Corp. of Virginia (Portsmouth. Va.). This paint,
described as product number TT-E-490, is said to meet Military
Specification Mil-C-83286B. After being painted in this manner,
each panel was again allowed to dry, this time for about 72 hours,
before being tested.
Bonding of the paint to the primed silicone rubber was evaluated by
a standard ASTM D-3359 crosshatch adhesion test. Briefly stated,
this procedure consists of scribing a square grid onto the coated
substrate with a special cutter, applying an adhesive tape to the
scribed area and peeling the tape off. The adhesive tape tends to
pull off the coating and the amount of the scribed coating
remaining is an indication of the integrity of its bond to the
substrate beneath. For the purposes herein, the following rating
scheme was used:
______________________________________ Rating Percent of Scribed
Surface Removed By Tape ______________________________________ 0
.sup..about. 100 (i.e., essentially no adhesion) 1 .sup..about. 80
2 .sup..about. 60 3 .sup..about. 40 4 .sup..about. 20 5
.sup..about. 0 (i.e., approximately 100 adhesion)
______________________________________
The results of these adhesion tests are presented in Table 1, from
which it can be seen that primer compositions not within the
critical range of components (I), (II) and (III) of the present
invention had ratings of no more than 3+ (corresponding to about
35% of paint removed by tape) while the primers of the invention
all had ratings of at least 4- (corresponding to about 25% of paint
removed by tape). It is further noteworthy that the aforementioned
composition of Fey et al. (expressed in molar ratios in Comparative
Example 25), had an adhesion rating of only 2.
TABLE 1
__________________________________________________________________________
Primer Mixture Mole Ratio Component Component Component of
Components Adhesion (I) (II) (III) (I):(II):(III) Rating
__________________________________________________________________________
Example 1 ATMS TBT DBTDA 52:24:24 5 2 " " " 58:24:18 4+ 3 " " "
42:39:19 4- 4 " " " 41:30:29 4+ 5 " " " 41:20:39 4+ 6 " " "
27:60:13 4+ 7 " " " 29:36:35 4 8 " " " 27:13:60 4 9 " " DBTDL
57:28:15 5 10 " TPT " 55:31:14 4 11 " " DBTDA 49:28:23 5 12 Z-6030
TBT " 41:30:29 4 13 MTMS " " 56:22:22 5 14 Z-6040 " " 42:29:29 4 15
VTMS TPT DBTDL 57:30:13 4 (Comparative) Example 16 ATMS TBT DBTDA
83:9:8 3 17 " " " 92:5:3 3 18 " " " 60:29:11 3+ 19 " " " 61:11:28 3
20 " " " 76:12:12 2-3 21 Z-6020 " " 44:28:28 1* 22 VTMS -- --
100:0:0 1 23 -- -- DBTDA 0:0:100 1 24 -- TBT -- 0:100:0 2 25 IBTMS
TPT DBTDA 92:6:2 2 Control -- -- -- -- 0
__________________________________________________________________________
*Formed a precipitate.
EXAMPLE 26
In a procedure similar to that described for Examples 1-25, a 100%
solids primer was prepared (i.e., one containing no solvent) having
a mole ratio of ATMS:TBT:DBTDA of 52:24:24. This primer was applied
to the silicone rubber substrate, as described above, allowed to
air dry for 15 minutes and then coated with the TT-E 490 Navy Grey
paint. This coating was air dried for seven days. An adhesion
rating of 5 resulted.
COMPARATIVE EXAMPLE 27
A primer mixture consisting of ATMS, titanium acetylacetonate and
DBTDA in a molar ratio of 52:24:24 was diluted to form a 12%
dispersion in heptane. The titanium acetylacetonate used was an
isopropyl alcohol chelate obtained from E. I. du Pont (Wilmington,
Del.) under the trade name of TYZOR.TM.TAA. This primer composition
formed a precipitate and could not be readily applied to the
silicone rubber substrate.
EXAMPLE 28
The above procedures were repeated wherein a peroxide-cured
silicone rubber, based on a copolymer of a polydimethylsiloxane gum
having about 0.14 mole percent of methylvinylsiloxane units and
filled with about 31 weight percent of a pyrogenic silica having an
approximate surface area of 255 m.sup.2 /g, was wiped with a primer
having the composition shown in Example 1 (diluted to a 11.9%
dispersion in heptane). After drying for about 30 minutes, the
primed silicone rubber was overcoated with the above described TT-E
490 and with a commercial spray paint (KRYLO.TM. Crystal Clear 1301
acrylic; Borden Inc., Columbus, Ohio). Upon further drying, these
coated substrates were subjected to the crosshatch adhesion test
and resulted in ratings of 5 in each case. Similarly painted
unprimed controls using this rubber substrate each had an adhesion
rating of 0.
COMPARATIVE EXAMPLES 29-30
The experiment of Example 28 was repeated using two room
temperature vulcanizing (RTV) silicone compositions, each of which
was based on a vinyl-terminated polydimethylsiloxane liquid,
methylvinylcyclopolysiloxane and a polyfunctional SiH-functional
crosslinker. The first of these RTVs was filled with about 12.4
weight percent of a pyrogenic silica having a surface area of about
255 m.sup.2 /g while the second RTV contained about 38 weight
percent of a mixture of 65 percent by weight of dimethylvinylsiloxy
endblocked polydimethylsiloxane oil and 35 percent by weight of a
benzene soluble resin copolymer of triorganosiloxy units and
SiO.sub.2 units having a ratio of about 0.7 mole of triorganosiloxy
unit per mole of SiO.sub.2 unit where the triorganosiloxy units are
trimethylsiloxy units and dimethylvinylsiloxy units and the resin
copolymer has about 2 weight percent silicon-bonded vinyl radicals.
Each of these RTV compositions was catalyzed with platinum, coated
onto a steel panel and cured.
A primer having the composition shown in Example 1 was applied to
each of the above cured silicone RTV substrates, dried for 30
minutes and overcoated with the TT-E 490 and the KRYLO.TM. paints.
After the paints were dried for 72 hours, each crosshatch adhesion
test resulted in a rating of 3 for the case of the first silicone
RTV substrate and a rating of 0 for the second silicone RTV
substrate. The unprimed controls gave ratings of 0 in each
instance. This series of experiments indicates that the unique
adhesion promoting character of the instant primer compositions is
not of universal utility for all silicone substrates. Rather, it is
specific to at least the types of silicone surfaces illustrated in
Examples 1-15, 26 and 28.
* * * * *